We have been investigating casposons, proposed to be novel mobile elements that may have been the precursor of bacterial and archaeal adaptive immune systems. When we first examined the DNA sequences surrounding these proposed mobile elements, we discovered that many of them are enclosed within short, duplicated segments of genomic DNA of 14-15 basepairs (3). This was new and compelling evidence that another hallmark of DNA transposition - target site duplications - was also associated with casposons. We have expressed and purified the proposed transposase associated with one particular casposon, that from a deep sea vent archaeal species, Aciduliprofundum boonei. We showed that this casposase can integrate both short oligonucleotides and an excised mini-casposon into target DNA. Casposon integration occurs without target specificity into pUC19 and generates 14-15 basepair target site duplications (4), consistent with those found in casposon host genomes. It has more recently been reported that, in the presence of the host tRNA-Pro gene, integration is overwhelmingly site-specific and insertion occurs into a particular site at the 3'-end of the tRNA-Pro gene (5). These collective results serve as the basis for our ongoing studies to structurally characterize casposases bound to their DNA substrates as a way to determine their mechanisms of action. 3. Hickman and Dyda (2014) CRISPR-Cas immunity and mobile DNA: a new superfamily of DNA transposons encoding a Cas1 endonuclease. Mobile DNA 5, 23. 4. Hickman and Dyda (2015) The casposon-encoded Cas1 protein from Aciduliprofundum boonei is a DNA integrase that generates target site duplications. Nucleic Acids Res. 43, 10576-10587. 5. Beguin, Charpin, Koonin, Porterre, and Krupovic (2016) Casposon integration shows strong target site preference and recapitulates protospacer integration by CRISPR-Cas systems. Nucleic Acids Res. 44, 10367-10376.